The invention relates to an audio processing device and a receiver for receiving and filtering a useful signal that has a spectral envelope situated in the voice band, and for restoring the useful signal in the presence of ambient noise.
The invention also relates to a telephone equipment comprising such a receiver.
The invention finally relates to a filtering method and a method of receiving a useful signal that has a spectral envelope situated in the voice band, for modifying the spectral envelope of the useful signal before restoring the useful signal in the presence of ambient noise.
The invention has many applications in electronic audio devices that may be used in a noisy environment. The invention is notably applied to mobile radiotelephony equipment that may be used inside a car and enables to reduce the acoustic annoyance linked with the noise of the engine and/or of the car radio.
United States patent published under U.S. Pat. No. 4,052,720 describes a dynamic ambient noise control system for distributing a random ambient noise at a workplace having surround sensors and for adjusting the spectrum and the amplitude of the distributed ambient noise as a function of the value of certain surround parameters measured at predetermined time intervals by the various sensors.
This system is intended to have a positive effect on users at their workplace, notably for enhancing their productivity or reducing the effect of interference between the various conversations. It shows the main drawback of increasing the sound level of the overall ambient noise, which, over a long period, may generate a specific tight feeling with the users.
It is an object of the invention to provide an audio processing device, a radio receiver and a filtering method and a receiving method for improving the acoustic comfort of the user in a noisy environment, even in the case of intensive and/or prolonged use.
For this purpose, the invention provides an audio processing device of the type defined in the opening paragraph, characterized in that it comprises means for tapping ambient noise, spectral envelope extraction means for extracting parameters of the tapped ambient noise envelope and digital filter means controlled by said envelope parameters for modifying the spectral envelope of the useful signal to be restored, said filter means comprising a digital filter having coefficients that can be parameterized, and co-operate with said envelope extraction means for parameterizing the filter with the aid of the envelope parameters.
According to a characteristic feature of the invention, a device as already mentioned is provided, characterized in that it comprises an echo canceling loop controlled by the output signal of the digital filter for suppressing the acoustic echo that exists in the sampled ambient noise and for supplying an estimate of the ambient noise to said extraction means. Similarly, the invention provides a filtering method as mentioned in the opening paragraph, characterized in that it comprises the following steps:
an acquisition and spectral analysis step of tapping the ambient noise, extracting an estimate of a spectral envelope therefrom and of deriving envelope parameters therefrom,
a step of calculating a time average between the envelope parameters to obtain average parameters called control parameters,
a parameterizing step of parameterizing a digital filter with the aid of said calculated control parameters, and
a filtering step of filtering the useful signal with the aid of said digital filter.